Ultraviolet curable ink is widely employed in the field of offset printing, gravure printing, screen printing, relief printing and ink-jet printing, due to quick drying, excellent ecological suitability due to an absence of volatile solvents, and the capability of printing onto various base materials.
In recent years, flexographic printing has been the focus in technical improvement of high image quality and suitability for short printing runs, compared to gravure printing and offset printing. Ink-jet printing has also been the focus in capability of on-demand printing and employing ultraviolet ink (hereinafter, referred to as also UV ink) thereby has resulted in not requiring a special medium for printing.
In order to apply non-solvent UV ink to flexographicgraphic printing or ink-jet printing, lowered ink viscosity is an essential requirement. Specifically in ink-jet printing, there is a problem in selecting inks having a viscosity at ejection from 2-30 mPa·s, and preferably from 3-20 mPa·s, even when using a piezo type head having relatively wide flexibility for various inks.
Further, in ink-jet printing, using pigments as coloring agents is preferred from the viewpoint of actinic radiation curability and image durability, however, problems of clogged nozzles occur in cases when the pigment particle size is more than 1 μm.
Examples of non-solvent UV curable inks are disclosed in JP-A Nos. 5-214279, 5-214280 (hereinafter, the term JP-A means Japanese Patent Application Publication), Toku Hyo 2000-505778 (Toku Hyo: Kohyo Tokkyo Koho=International Patent Application Publication; hereinafter, referred to as JP-C), WO 99/29787, WO 99/29788 (hereinafter, the term WO means International Patent Publication). A method to dilute the curable ink with polymerizing monomers after dispersion of pigments in conjunction with a dispersing agent to prepare a pigment mill base to heighten the pigment dispersibility, is disclosed in WO 99/29787 and WO 99/29788. On the other hand, solid colored compounds for water based ink-jet ink and preparation methods for the same are disclosed in JP-A 2002-12810.
As mentioned above, to obtain excellent colorant dispersion it is effective that blending and dispersion is conducted in a state of high pigment ratio, on the condition that pigments and a dispersing agent are mixed beforehand.
In these methods, however, if a resin for colorant dispersion is selected considering only dispersibility, there is a problem of impaired printing suitability due to deterioration of ink storage stability, lowered curing sensitivity and deterioration of ejection stability due to an increase in ink viscosity caused by resin components.
Further, if a low viscosity polymerizable compound is employed as a dispersion medium, there may be problems in that polymerization proceeds when dispersion is conducted to obtain sufficient dispersibility.